The damaging effects of low temperatures on fresh concrete are basically twofold. Pure water freezes at 0°C and this causes its volume to increase by approximately 9%. If this occurs, and the concrete does not yet have sufficient tensile strength to withstand the stress generated by the freezing of the internal water, there will be irreversible damage that will limit the mechanical performance and durability of the concrete. In this sense, concrete is considered to be resistant to the effects of cold weather (due to expansion of the frozen water) when it has reached approximately 3.5 N/mm2.
The second problem is related to the cement hydration rate and is closely linked to the first problem. Low temperatures act as a setting retarder, slowing down the whole process of strength development (to reach the minimum value at which the concrete can already face a frost). The relevance of this problem will be even greater if cements evolve according to the logic of trends (in order to reduce emissions), resulting in cements that are more sensitive to the retarding effect of low temperatures.
All regulations in all advanced countries related to the execution of concrete structures include conditions and regulations for concreting in cold weather. In all of them, a series of compulsory measures and boundary conditions are listed to guarantee the quality of the structures when concreting in cold weather, including the use of anti-freezing additives and setting accelerators.
However, in addition to the instructions in the respective standards for concreting in cold weather, there are other important considerations that are not always taken into account. The aim of all of these is to ensure that the concrete has developed sufficient strength by the time the frost arrives.
The most important admixture when concreting in cold weather is the water-reducing admixture.
When concreting in cold weather, the first thing that comes to mind is anti-freeze admixtures. However, the importance and role of plasticising admixtures, due to their setting retarding character, is rarely known. This point is particularly relevant in the case of ready-mixed concrete delivered by truck, where such admixtures are used on a regular basis.
The retarding effect of conventional plasticising admixtures increases exponentially with decreasing fresh concrete temperature, so in cold weather conditions they should be used with caution. In low temperature situations, it is highly recommended to minimise the dosage of plasticising admixture to the maximum (<0.4% spc) and increase the dosage of superplasticising admixture until the desired consistency is obtained. With this practice, the onset of setting of the concrete is advanced and the development of initial strengths is accelerated (with hardly any practical consequences on the maintenance of consistency during transport), which is very beneficial for the concrete to be able to develop maximum strength when the negative temperatures arrive.
The key is concreting at the optimum time of day.
On a cold weather day, we could say that the lowest temperatures are recorded from dusk until late morning. During this time it is possible for the temperature to be below 0°C for many hours, leaving the young concrete exposed to possible damage. Consequently, if by the time these conditions occur the concrete has not been able to build up sufficient strength to resist the effects of frost, damage is inevitable and irreversible.
The concrete should always be placed as early in the day as possible after the minimum permissible concreting temperature has been exceeded. The aim is to allow the fresh concrete to spend as many hours as possible in the maximum temperature range so that it sets as quickly as possible and can develop strength for as long as possible (below +4°C hydration hardly progresses at all). This is probably the most effective measure for safe concreting in cold weather.
Anti-freeze admixtures are used in cold weather situations to accelerate the onset of setting and the development of initial strengths during the time when, after laying, temperatures are favourable for cement hydration to progress. These additives allow concreting to be carried out in conditions in which, without their use, the young concrete would not be able to develop the minimum sufficient strength to face the moment of frost with guarantees. This is the case on winter days when the maximum temperature does not exceed 5-6°C, for example, or when low temperature is combined with very high humidity. However, it is important to note that these are not miracle products, so the indications of the regulations in force must always be respected.
If at the time of the first frost the concrete has been able to develop sufficient strength to withstand the stress generated by the freezing of the water, it does not matter what minimum ambient temperature can be reached, as the physical effects are the same at -3°C as they are at -10°C.
It might be thought that the optimum time for concreting in cold weather would coincide with the maximum temperatures of the day, but this is not the case. For example, concreting at 16:00 h would be foolhardy despite being in the peak temperature range, as the concrete would not have enough time to develop sufficient minimum strength by the time the temperature starts to show negative values.
Using the internal heat of fresh concrete as a setting accelerator
In cold weather conditions (for concrete purposes), the time when fresh concrete is at its highest temperature is undoubtedly just after mixing. The formation of ettringite in the immediate phase of cement hydration and the energy of the mixing itself generate heat which accumulates in the freshly mixed concrete. In addition, the concrete plant has the means to ensure the required minimum fresh concrete temperature in cold weather conditions.
From this point onwards, the internal heat accumulated in the freshly mixed concrete will dissipate over time. Therefore, it is important to minimise the time between the mixing of the concrete and the end of placing. Running concrete a few degrees warmer is critical to accelerate the development of early strength. To this end, in cold weather conditions, good on-site preparation with the aim of applying the concrete as quickly as possible reduces the risks typical of cold weather on young concrete. In the same vein, it is preferable that the transport time between the plant and the construction site be as short as possible.
Not all structures are the same in cold weather
In line with the previous point, the internal heat dissipation of freshly mixed concrete continues once it has been cast (until the start of setting, when internal heat will develop). As a general rule, the larger the volume of concrete, the lower the risks against cold weather conditions, but also the internal heat loss is closely linked to the geometry and the exposed surface area over which the internal heat of the particular structure can be dissipated.
Structures that are slender or have a large exposed surface area, such as a pavement, will lose the accumulated heat more quickly than a structure that accumulates the largest volume with the smallest exposed surface area or is directly protected (buried structures, formwork).
When concreting in cold weather, it is always advisable to insulate and protect the newly concreted structures as much as possible to preserve the internal heat, but this measure is especially critical when unfavourable structures in terms of heat dissipation have to be built.
